Ryugu and Bennu are
small targets. Ryugu’s
polar diameter is about
880 meters and Bennu’s
is about 510 meters.
Both asteroids orbit the
sun on trajectories that
sometimes take them
relatively close to Earth.
SOURCE: UNIV. OF ARIZONA
... with similar orbits
Shogo Tachibana greeted asteroid Ryugu with dread. The cosmochemist with the University of Tokyo had spent 10 years helping to design
a mission to Ryugu’s surface. To touch down safely,
the spacecraft, Hayabusa2, needs to find broad,
flat stretches of fine-grained dust on the asteroid.
But on June 27, when Hayabusa2 finally reached
its target after a three-and-a-half-year journey
(SN Online: 6/27/18), Tachibana got a rude awakening: Ryugu is covered in boulders. Big ones.
“We cannot find a 100 percent safe place to
touch down,” Tachibana says. “It seems to be a
very dangerous place.”
If Hayabusa2 can deal with the boulders — and
any other challenges that arise — it will become
only the second spacecraft to bring a piece of
an asteroid back to Earth. And the mission will
answer questions that its predecessor couldn’t.
The original Hayabusa mission visited a sand-
and rock-covered asteroid called Itokawa in 2005.
But Itokawa has the wrong chemical makeup to
address big questions about the origin of life that
Ryugu, which is carbon-rich, is well suited for.
And Hayabusa suffered a series of calamities that
caused it to return to Earth several years late, with
less than 2,000 grains of precious asteroid dust.
Tachibana and colleagues from the Japanese
Aerospace Exploration Agency, or JAXA, are
counting on Haybusa2 to return bits of Ryugu’s
surface to Earth in 2020. And if a daring plan to
blow a crater into the asteroid works, the spacecraft will get some subsurface grains as well.
A sister project from NASA, the OSIRIS-REx
mission, arrived at an asteroid called Bennu
in December to bring samples back in 2023
(SN Online: 12/3/18).
The two spacecraft face daunting challenges.
The probes must investigate objects that have
so little gravity that sunlight can knock them
off their orbits. If the probes manage to pick up
samples, the spacecraft must keep the dust pristine during the trip back to Earth. To get the most
out of the missions, the Japanese and American
teams are trying to work together across cultural
and bureaucratic divides.
But the uncertainties and anxiety are worth it.
Asteroids like Ryugu and Bennu are among the
oldest and most intriguing objects in the solar
system. They could hold the keys to some of the
most pressing planetary questions: What came
before the planets? What are the origins of life?
And how much of a threat do asteroids pose to life
on Earth today?
Of course, planetary scientists already have
tens of thousands of asteroid pieces to study.
Such meteorites fall to Earth in the hundreds
each year, offering researchers plenty of material to slice, grind and examine for clues to the
solar system’s history.
Dante Lauretta of the University of Arizona in
Tucson, the principal investigator of OSIRIS-REx,
spent the first part of his career trying to coax
meteorites into telling him whether molecules
necessary for life — such as nucleic acids, amino
acids and phosphorus, which are structural components of DNA — could have originated inside
carbon-rich asteroids like Ryugu or Bennu.
Carbon-rich asteroids are thought to be mostly
unchanged since their formation at least 4. 6 billion
years ago, which makes them perfect time capsules.
A few grains of such an asteroid could reveal what
the early solar system was made of.
Remote studies of asteroids also suggest that the
raw ingredients for life, and maybe even the chemical processes that are necessary for life to begin,
might have been present on carbon-rich asteroids
even before the planets were done growing.
“We think an asteroid like this one may have
delivered this material to the surface of the early
Earth, providing seeds or building blocks of life,”
Lauretta says. “If we can show the precursors [of
life] started before the planet, I think the prob-
ability that there’s life elsewhere in the solar
system goes way up.”
Studying meteorites to explore this notion
falls short on two fronts, however: It’s hard to tell
where they come from, and they’re contaminated.
As soon as a space rock hits Earth’s atmosphere, it
starts accumulating signs of Earth life. Therefore,
any intriguing organic compounds in a meteorite
could be from Earth, not native to the asteroid.
There’s no way to tell.
“We needed samples of a carbon-rich asteroid
to really answer the questions I was into,” Lauretta
Break me off a piece
Getting to the origins of the solar system, and
maybe life’s beginnings, makes bringing clean,
carefully selected samples to earthly labs crucial.
But spacecraft can’t just dig in with a shovel.
There’s no grabbing a rock with a claw like in an
arcade game. The asteroids are so tiny — Ryugu is
about 880 meters from pole to pole and Bennu is
about 510 meters — and their gravity so weak that
reaching out and grabbing something could push
the spacecraft off course with the asteroid.